The "MT-32" emulation of the SC-55 based modules is pretty worthless for games. What it does have is an alternate instrument map to match the default mapping of the MT-32. While that sounds useful, it misses a major feature of the MT-32 and that is the ability to dynamically reprogram the synthesizer to produce different "instruments" than the default set and the SC-55 is incapable of doing that since its synthesis is sample based instead. A lot of old games use this feature and will not sound anywhere near right on the SC-55 in MT-32 mode.

Yes, I know about that. The problem is, I'm not that kind of purist to hunt down MT-32 - I'm already happy with my SC-155 and a LOT of games use it (it's SC-55 with additional functions). MT-32 era games is not my prime point of intereset - it's kinda nice to use original hardware even in inferior sound-mode (through emulation), but nothing short of "ok" - better this than no sound at all or PC Speaker. Probably I buy somewhere in the future original hardware. I was using VSTi Roland Sound Canvas and Yamaha XG software emulation for MIDI playback till now and it's kinda nice to use original hardware - I have to start somewhere so SC-155 is good entry-point. Terratec Phase 26 USB is very good audio interface for audio playback (Windows-only), so my main concern is about soundcard and connecting it to Roland.

I think you would be better off installing windows 98. Windows 98 can play DOS, early windows, and early XP games just fine. You also won't have any trouble with external midi devices, emulated or otherwise. There are lots of great sound cards for windows 98 with the features you want. A SB Live or an Aureal A3D card would be great for windows, an ESS or Yamaha card would do just fine for DOS.

As for the voodoo cards, I think a voodoo 3 makes the most sense. Compatibility with DOS games is pretty excellent, although you will have to jump through some hoops to get some of the DOS games that support glide to work properly with a voodoo 3. The hardware that would suit your use case best is an super socket 7 board with a K6 3+ processor. Unfortunately, the boards and processor have become quite expensive as they are sought out by collectors.

https://www.youtube.com/watch?v=fcAqRbFFQPU

You could also pair a voodoo 3 card with a much less expensive pentium 3 system or even that duron 1800+, it just wont' be as compatiable with speed sensitive dos games, and finding boards with ISA slots becomes more difficult.

Like you said though, voodoo cards are expensive these days, and I can't blame you if you don't' want to spend a lot of money on an old voodoo card.

Another option is to use a glide wrapper like nglide or dgvoodoo 2 to emulate the glide API on modern hardware. The emulation is pretty excellent, and you get lots more performance, 32 bit colors, AA and AF filtering, higher resolutions, ect. Compatibility with old games and new operating systems is also really great. You can even use nglide with DOSbox.

I would just use that pentium MMX machine with your voodoo 1 for DOS games, anything before 1998 would work at least ok. For windows games, I would recommend GoG and a glide wrapper. In fact, a lot of GoG games come with nglide already installed. To make use of glide wrapper, you will need at least XP and a card that support directx 9c (so anything geforce FX or later, ideally an 8800gt or better).

Windows games did not make use of external midi devices, instead focus shifted to hardware acceleration like A3D and EAX. A more modern computer and a DOS computer would be cheaper and arguably better than SS7 build or a pIII.